As one of the means for decreasing the cost of a power semiconductor module, it is possible to fabricate the package by transfer molding as in the fabrication of an IC package. Power semiconductor modules having cross-sectional structures as shown in FIG. 3 and FIG. 20 are typical examples.
FIG. 3 shows an IGBT module incorporating a driver (three-phase inverter module) produced by mounting six elements, each including an IGBT and flywheel diode (FWD), and four chips having a gate drive IC (incorporating various protecting circuits) for the IGBT, on a lead frame 11 and subjecting the same to transfer molding. The bare chip power elements (IGBT, FWD) 14 and the bare chip gate drive ICs 30 are mounted on a common lead frame 11 serving as both a power system terminal 12 and a control terminal 13, and electrical interconnections are made with Al (aluminum) wires 15, and then the same is subjected to transfer molding (first molding) with a thermosetting resin 31. Then, the same and a heat radiating plate 10 together are subjected to transfer molding with a thermosetting resin 32 (second molding). Further, insulating work between the lead frame 11 and the heat radiating plate 10 is performed with the thermosetting resin 32 at the same time as the second molding. Hence, a resin containing a large quantity of alumina filler is used as the thermosetting resin 32 for reducing the heat resistance.
According to the IGBT module as described above, the only parts used other than the Si chip are the lead frame 11, the heat radiating plate 10, and the sealing resin. Hence, it has an advantage of being fabricated at low cost.
FIG. 20 shows an example of a hybrid IC mounting power elements thereon. Namely, it shows a technique for attaining a performance higher than the IGBT module incorporating a driver shown in FIG. 3. There are mounted bare chip power elements 14 and a thick film circuit board (alumina) 202 on a lead frame 200. A flip-chipped IC 201 is mounted on the thick film circuit board 202, whereby a thick film resistor and the like and a high performance circuit are formed. The power element 14 and the thick film circuit board 202 are connected to the lead frame 200 with an Al wire 15 and subjected to transfer molding with a thermosetting resin 16. Since, as opposed to the case of FIG. 3, this example uses the thick film circuit board 202 in addition to the lead frame 200, it has a characteristic feature of attaining a higher performance.
The above described transfer mold type power modules have the following problems in terms of compatibility between achievement of high performance of the module and reduction in the fabrication cost and, further, in terms of reliability of the module.
In the case of the IGBT module shown in FIG. 3, the electrical wiring pattern is provided by only the lead frame 11. Hence, the number of components can be decreased and the fabrication process can be greatly simplified. It, therefore, has a structure well designed in terms of fabrication cost. However, in order to lower the resistance of the power system terminal 12, it is impossible to decrease the thickness of the lead frame 11 and, therefore, a fine pattern of the lead frame 11 cannot be produced. Accordingly, the control circuit that can be mounted is limited to a circuit not requiring a fine pattern. Namely, it is limited to such a circuit as an IGBT module incorporating a driver mounting only a gate drive IC thereon as in the case of FIG. 3. Further, since it is not necessary to consider heat radiation therefrom, the gate drive IC 30, in itself, is a part not required to be mounted on the lead frame 11. Hence, this means that the area of the module increases by a region occupied by the gate drive IC 30. This is unfavorable from the point of view of fabrication cost when transfer molding, to which miniaturization is the key, is executed.
On the other hand, in the case of the structure shown in FIG. 20, a fine pattern can be easily produced for the control circuit because the thick film circuit board 202 serving as the control circuit is mounted on the lead frame constituting the power system terminal. Hence, a high performance IC such as an MPU can be incorporated therein. However, the area of the lead frame 200 becomes large and there arises the same problem as the case described above in view of the fabrication cost.
Further, as a problem common with FIG. 3 and FIG. 20, it can be mentioned that they are both vulnerable to noises. Since both the control circuit and the power element are mounted on the same lead frame, capacity coupling is caused between the power element 14 and the control circuit (the gate drive IC 30 or the thick-film circuit board 202) through the lead frame or the radiating plate and the circuit is easily affected by noise due to potential changes in the power element 14.